Atmospheric conditions that are important to designers of airborne vehicles include liquid water content (LWC) and ice water content (IWC). While LWC has for some time been the focus of sources of hazardous ice due to the freezing of supercooled liquid water on aircraft surfaces, attention recently has been directed to the ice content of the atmosphere and potential hazardous accumulation of ice particles. These particles may be in the form of individual ice crystals, aggregates of crystals such as snowflakes, or crystals that have collided with supercooled water droplets to form more dense and spherical particles such as graupel and hail. The size of ice particles can vary significantly, from microns to centimeters. In the past, ice particles generally were not considered to be a hazard to the outside shell of an aircraft because they will typically bounce off the surface of the aircraft and not accumulate. Ice particles can nonetheless cause problems associated with an aircraft. For example, when ice particles are ingested into an aircraft's engines, ducts, or cavities, they can collect together and form a blockage that can be detrimental. Also, accumulated ice particles can melt and then refreeze within a downstream portion of the aircraft's systems, further causing problems to engines and/or air data instrumentation. To help assess these potential problems, the FAA has created an Ice Protection Harmonization Working Group that among other activities investigates engine events attributed to ice particle ingestion.
A need exists in the current state of the art to detect the accumulation of ice particles, and to detect ice particles that are present in sufficient concentration for a sufficient time period, since a substantial concentration of ice may have a hazardous impact on airborne vehicles.